The present invention relates to fuel cells and, in particular, to a method for activating solid polymer electrolyte type fuel cells.
In recent years, with the social trend for the growing concern about the environmental protection problems, the development on the solid polymer electrolyte type fuel cell (hereinafter, to be briefly referred to as "PEFC") is under remarkable progress in the field of the fuel cells. Although the fuel cells are in their step of just being to put to practical uses, they are still not satisfactory due to many reasons.
The conventional PEFC is configured by including a proton conductive polymer electrolyte membrane, a pair of positive and negative electrodes, bipolar plates made of carbon or metal, and cooling plates. Each electrode is configured by including a mixture of carbon powder with catalyst powder such as Pt. And, if required, a water-repelling agent such as a fluorocarbon compound is added to the mixture. The electrode is configured by joining on a gas-diffusing layer and the electrode is combined with the proton conductive polymer film. In a case of using pure hydrogen as the fuel gas, it is possible to use the same material for configuring both the positive electrode and the negative electrode.
(1) Problems due to the fuel.
Usually, fuel gas or a fuel gas obtained by reforming of methanol or methane gas is used for the fuel cells. However, in the particular case of PEFC which usually employs a platinum catalyst in the electrode, there is a problem of poisoning the platinum catalyst by carbon monoxide in fuel gas, thereby decreasing the catalytic activity and inviting a serious deterioration in the cell performance.
In order to avoid this problem, there have been proposed various methods. As one of them, there is a hydrogen separating method, whereby the carbon monoxide in the fuel gas is removed by the use of a Pd thin membrane in advance of the introduction of the fuel gas into the PEFC. In this method, hydrogen is selectively caused to pass through a Pd membrane selectively by applying a certain pressure at one side of the thin membrane. This method is already used in a plant for manufacturing semiconductor devices or the like, and is under development also for the PEFC.
As another method for decreasing the CO concentration of the fuel gas, a so-called CO-denaturing method is proposed. In this method, after reforming methanol or methane gas, the CO was removed from the reformed gas by the use of a CO-denaturing catalyst (CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2). By this method, it is usually possible to decrease the CO concentration of the reformed gas down to 0.4 to 1.4%. If the CO concentration can be decreased to this extent, the reformed gas can be used for a phosphoric acid type-fuel cell which also employs the same Pt electrode catalyst. However, in order to prevent the possible poisoning of the platinum catalyst in PEFC, the CO concentration should be decreased down to a level of at least several ppm, and thus the above-mentioned CO-denaturing method is still not satisfactory.
Under the stated circumstances, another method is proposed for further removing the CO in the CO-denatured gas by further introducing oxygen (air) into the CO-denatured gas, thereby oxidizing the CO by the use of an oxidizing catalyst at 200 to 300.degree. C. For the oxidizing catalyst used in this method, an alumina catalyst which carries a noble metal is proposed, for example. It is however very difficult to selectively and completely oxidize the CO in the hydrogen.
Further, although various investigations of an alloy catalyst are conducted to have higher resistance against the poisoning by CO, the performance of such an electrode catalyst is not satisfactory and, thus, it is difficult to develop an electrode catalyst which does not completely adsorb CO.
Moreover even if, the CO-oxidizing method and the method of mixing air with the fuel gas are employed, it is difficult to sufficiently decrease the CO concentration down to the extent for the PEFC. There is a hazard of including a large amount of CO at a start-up stage of the fuel cell. Thus, there is a need for a long time period in order to be stabilized before introducing the fuel gas, or a need for separately providing a hydrogen reservoir (bomb) solely for the start-up. In addition, the CO is gradually accumulated in the fuel electrode even under normal operating conditions and the cell performance is gradually deteriorated. Once the cell performance has been deteriorated, it cannot be recovered automatically, and there is a need for removing the accumulated CO by oxidizing the CO by temporarily suspending the operation of the fuel cell and introducing a large amount of air, or replacing the whole electrode assembly with fresh one.
(2) Problems due to the water-repelling property of the polymer electrolyte membrane
Incidentally, a compound having a main chain (repetitive unit) of --CF.sub.3 -- and a side chain containing sulfonic groups (--SO.sub.3 H) at the end functional group is generally used as the proton conductive polymer electrolyte. This type of electrolyte has a proton conductivity with water, which must be supplied from the outside. For that reason, the electrolyte must be constantly attached (in contact) with water under the operating condition of the cell. But the electrolyte has a strong acidity with containing water. Material of any parts and components of the cell, which are in direct contact with the electrolyte, should therefore have acid resistance.
Since the polymer electrolyte needs water, in a case of operating the PEFC, it is required to humidify the fuel and the air to a dew point before they are supplied to the cell. In particular, the higher the operating temperature of the cell is, the more important becomes the humidity control on the supplying gases.
In a case of loading a PEFC to operate just after the assembly, or in another case of loading the PEFC again which had been standing still in non-operated state for a long period, it is generally difficult to immediately obtain sufficient performance. The cause for this phenomenon is due to the fact that a long time is required for hydrating an electrode diffusing layer completely, because the electrode diffusing layer of the PEFC has been treated for water-repelling.
In addition, a long time is also required for sufficiently wetting the material, which is the same case of polymer electrolyte, contained in the electrode catalyst. Further, even if the cell is kept to a moderate temperature and the gases, which are adjusted to a moderate temperature and humidity, the electrode diffusing layer is hard to hydrate when the cell is left in the no-loaded state. Moreover, the material contained in the electrode catalyst is hard to humidify and, thus, it becomes hardly possible to derive the sufficient cell output unless the cell is continuously subjected to generate the electricity generation at a high current density for several days.
For the reasons mentioned above, in order to derive the cell with the high performance at an early stage, an activating treatment was conventionally practiced by, for instance, generating electricity at a higher current density with a pure oxygen, or by maintaining the cell voltage at about 0 V by regulating the potential while supplying a large amount of the fuel gases. Even with these methods, there is still a problem to be solved that a time period of more than several hours is required to derive the cell with the high performance.
It is therefore the primary object of the present invention to solve the above-mentioned problems (1) and (2). More specific objects of the present invention are to provide an easy and highly effective method for activating the fuel cell by preventing the deterioration in the cell performance due to CO poisoning or restoring the deteriorated cell performance, and a method for activating the fuel cell by reducing the delay in demonstrating the cell performance due to the water-repelling property of the polymer electrolyte membrane.